1. Field of the Invention
The present invention relates to a multi-purpose sensor that is to be attached to the skin surface of the object for measuring more than one kind of vital sign.
2. Related Art
Various methods are conventionally used to measure pulse waves in the object or to monitor breathing. If one wants to analyze pressure waves obtained from the body surface non-invasively, they must be measured simultaneously with the recording of an electrocardiogram (hereinafter ECG if applicable). In this case, ECG electrodes are attached to the object but, at the same time, in order to record pulse waves, a pressure transducer such as a piezoelectric element or gage for converting pressure changes to electric signals must be placed over the arterial vessel of interest. In most cases, the pressure transducer is placed over either the right or left common carotid artery or radial artery. If pulse waves are to be detected on the common carotid artery, it is necessary to use a special apparatus for compressing the pressure transducer against the artery from the outside.
Respiratory movements provide important information in monitoring the functions of the patient's body in ICU and other care facilities and they can be measured by various methods including: impedance measurement with two-electrode or four-electrode systems; a belt-type respiratory pickup that detects changes in the resistance of an electrolyte in a tube that expands or contracts in response to thoracic movements; and a thermistor-type respiratory pickup that is attached to the nose for detecting temperature changes due to air flow during breathing.
The above-described methods have their own problems. First, in order to measure pulse waves with the pressure transducer placed over the common carotid artery, the subject must lie quietly on a bed or other supports. In the case of detecting pulse waves on the radial artery, a pressure transducer typically using a piezoelectric element must be attached to the wrist of the subject by winding a belt so that a certain pressure will be exerted on the site of measurement. As a further problem, the use of pressure transducers requires the finding of an appropriate site for attachment, which can only be done by a skilled technician. If the subject moves the wrist to which the sensor is attached, it may be displaced in position to be no longer capable of detecting pulse waves. Hence, this method also requires rest for the subject during measurement and it is extremely difficult to carry out the intended measurement while the subject is carrying on daily activities. What is more, the transducers under consideration are too bulky and expensive to be used as disposable products.
When using the impedance-measuring pickup to monitor respiratory movements, two or four electrodes are attached to the body surface and the changes in resistance that occur in the body tissues between electrodes in response to thoracic movements are detected as changes in respiratory movements. A problem with this approach is that in order to insure that small changes that occur between electrodes are detected as impedance changes, the spacing of two electrodes must be increased.
When using the belt-type pickup, a belt fitted with a tubular sensor is wound around the subject's chest and this gives the subject a feeling of constraint in that part of his body that is tightened by the belt. This is the primary reason why the belt-type pickup is not used very often today. Moreover, the clamping force of the belt must be within reasonable limits, but as a result, the belt is prone to displacement on account of body movements; therefore, measurement with the belt-type pickup also requires the subject to be at rest. It should also be noted that the pickup is too expensive to be used as a disposable product.
The major problem with the use of the thermistor-type pickup which is attached to the nose is the discomfort it causes to the subject. Further, the pickup consumes so much electric power that it is not suitable for use with battery-driven wireless telemeters.
As a further problem, if one wants to record an ECG simultaneously with the measurement of pulse waves or respiration rates, at least two ECG electrodes must be attached to the subject's body in addition to the transducer for measurement of pulse waves or respiration rates. This only adds to the constraint on the side of the subject, who feels greater discomfort or pain.